1. Field of the Invention
The present invention relates to a level shift circuit, and more specifically, to a level circuit with low sensitivity to a threshold voltage and adapted to be operated under low input voltage.
2. Description of Prior Art
With a rapid development of monitor types, novel and colorful monitors with high resolution, e.g., liquid crystal displays (LCDs), are indispensable components used in various electronic products such as monitors for notebook computers, personal digital assistants (PDAs), digital cameras, and projectors. The demand for the novelty and colorful monitors has increased tremendously. A Low Temperature Poly-Silicon Liquid Crystal Display (LTPS LCD) panel, on account of high resolution demands, is widely applied to various electronic devices.
Referring to FIG. 1 illustrating a functional block diagram of a conventional source driver, the source driver 10 comprises an output stage circuit 161, a digital-to-analog converter (DAC) 162, a level shift circuit 163, a latch 164, a data buffer 165, and a shift register 166. The shift register 166 is used for sequentially shifting an input impulse from an external circuit in response to a clock signal CLK. Data signals D00P/N-D02P/N, D10P/N-D102P/N, D20P/N-D22P/N are stored to the data buffer 165 through a bus based on the output from the shift register 166. The control signal STB is fed to the latch 164 and the output stage circuit 161. The latch 164 is used for latching the data signals D00P/N-D02P/N, D10P/N-D102P/N, D20P/N-D22P/N for a while and outputting the data signals D00P/N-D02P/N, D10P/N-D102P/N, D20P/N-D22P/N simultaneously. The level shift circuit 163 is used for raising voltage level of output of the latch 164. The DAC 162 can transform the digital data signal into an analog signal. While the control signal STB is at a raising edge, video data stream composed by the data signals is delivered from the data buffer 165 to the latch 164; alternatively, while the control signal is at a falling edge, the video data stream is fed to pixels of a liquid crystal panel via the output stage circuit 161.
Referring to FIG. 2, a functional block diagram of a conventional level shift circuit 163 is shown. At present, by using LTPS processes, circuits can be mounted on the glass. Nevertheless, it is a complex task for a designer to consider instabilities and variations among LTPS-manufactured elements, especially an impact of circuit quality attributed to threshold voltage (VTH) of a transistor and electrons mobility rate (μ). A conventional level shift 163 for raising voltage level of an input signal may occur a delay or erroneous operation owing to low-voltage input signal or poor transistor characteristics.